In recent years, aluminum alloy materials have been gaining attention as materials for vehicle structural members and the like from a global environmental viewpoint. For instance, when a product is manufactured using a heat treated aluminum alloy material comprising an Al—Cu—Mg base alloy, Al—Mg—Si base alloy, or Al—Zn—Mg base alloy, an aluminum alloy material is first formed into a desired shape by shape forming, which involves press shaping or the like. Next, solution treatment is applied to the aluminum alloy material subjected to shape forming in a manner such that a solid solution is formed with precipitation strengthening elements contained in the aluminum alloy material. Thereafter, precipitation is induced such that precipitates comprising, for example, Mg2Si are formed in the aluminum alloy material. Then, in order to harden the aluminum alloy material, an aging treatment is carried out at a temperature below the recrystallization temperature. However, in the above production process, an aluminum alloy material is heated above the recrystallization temperature during solution treatment. As a result, the strength of the aluminum alloy material decreases. Therefore, desired strength cannot be achieved in some cases, even if the alloy material is subjected to aging treatment.
In view of this problem of strength reduction, a process for producing an aluminum alloy material shown in, for example, FIG. 4 has been suggested. Specifically, a suggested process for producing an aluminum alloy material comprises at least the steps of: carrying out plastic forming (i.e., severe forming) by repeatedly imparting plastic strain to an aluminum alloy material (to which Zr or Sc that is able to form a thermally stable compound has been preliminarily added) under warm conditions prior to solution treatment; applying solution treatment to the aluminum alloy material subjected to plastic forming; and applying aging treatment to the aluminum alloy material subjected to solution treatment (see Non-Patent Document 1). According to the above production process, it is possible to inhibit recrystallization of an aluminum alloy material, which is caused during a solution treatment, by preliminarily adding Zr or Sc as an additive element to an aluminum alloy material. In addition, it is possible to miniaturize crystal grains in an aluminum alloy material by repeatedly imparting plastic strain to an aluminum alloy material under warm conditions. Thus, the material strength can be improved. Further, the plastic forming is carried out prior to solution treatment, which means plastic forming can be carried out during shape forming of an aluminum alloy material. Thus, plastic strain can be efficiently (in terms of time) imparted to an aluminum alloy material in a less.
Non-Patent Document 1: Tadashi Minoda et al., Crystal grain miniaturization of a 7475 aluminum alloy plate material by warm rolling, The Japan Institute of Light Metals, December, 2001, vol. 51, no. 12, pp. 651-655